Processes for the production of alkylene carbonates are known. WO-A 2005/003113 discloses a process in which carbon dioxide is contacted with an alkylene oxide in the presence of a suitable catalyst. The catalyst disclosed is a tetraalkyl phosphonium compound. This specification discloses that the catalyst is very stable if the catalyst is recycled to the alkylene carbonate preparation in an alcohol, in particular in propylene glycol (1,2-propane diol). In WO-A 2005/051939 it is disclosed that the decomposition of such a phosphonium catalyst is reduced if the reaction is conducted in the presence of a minor amount of carbonyl compounds, in particular aldehydes. Both documents show the effectiveness of the processes in batch experiments.
Although the presence of 1,2-propane diol as solvent reduces the decomposition of the phosphonium catalyst, it has the disadvantage that the compound is prone to reaction with the alkylene oxide. This becomes more apparent in the case of a continuous process in which the catalyst is recycled to the reactor where the alkylene carbonate is actually formed. Further, in a continuous process the reaction product containing alkylene carbonate, 1,2-propane diol and catalyst has to be subjected to a work-up treatment. Such work-up treatment generally includes one or more distillation steps to separate the product from the reactants. Since the boiling point of 1,2-propane diol is lower than that of propylene carbonate, 1,2-propane diol is removed from the propylene carbonate during the work-up of the reaction product. Therefore, the stabilizing effect of the 1,2-propane diol disappears during the work-up.